1. Field of the Invention
The present invention relates to an apparatus for supplying oil in engines. More particularly, the present invention pertains to an improved apparatus for supplying oil to a hydraulic variable valve performance mechanism and a structure for supplying lubricant oil.
2. Description of the Related Art
Many existing engines are equipped with a mechanism for varying characteristics, such as valve timing and lift, of a set of intake valves or a set of exhaust valves in accordance with the engine running conditions. This enhances the power and performance of the engine and reduces undesirable emissions. Japanese Examined Patent Publication No. 3-13403 discloses a variable valve performance mechanism that hydraulically changes valve lift and an apparatus for lubricating the moving parts in the variable valve performance mechanism.
As shown in FIGS. 6(a) and 6(b), a mechanism 71 for varying valve performance includes a rocker shaft 73, in which an oil pressure passage 72 is defined. A low speed rocker arm 74 and a high speed rocker arm 75 are pivotally mounted on the rocker shaft 73 in association with a valve. The rocker arms 74, 75 are pivoted about the axis of the rocker shaft 73 by a low speed cam and a high speed cam (neither of which is shown), respectively. Pivoting of the low speed rocker arm 74 about the axis of the rocker shaft 73 opens and closes the valve. "Right" and "left" as used below refer to the right and left directions of FIGS. 6(a) and 6(b).
A hole 76 extends in the low speed and high speed rocker arms 74, 75 parallel to the rocker shaft 73. A segmented coupling pin 77 is slidably fitted in the hole 76. An oil chamber 78 is defined between the right end of the pin 77 and the right end of the hole 76. The chamber 78 communicates with the oil pressure passage 72. A coil spring 79 extends between the left end of the coupling pin 77 and the left end of the hole 76.
When moved to a position close to the left end of the hole 76 against the force of the coil spring 79 as shown in FIG. 6(b), the coupling pin 77 couples the low speed rocker arm 74 with the high speed rocker arm 75. This causes the low speed rocker arm 74 to pivot integrally with the high speed rocker arm 75. As a result, the valve is opened and closed by the high speed cam by way of the low speed and high speed rocker arms 74, 75. This increases the valve lift. When the coupling pin 77 is moved to the right end of the hole 76 by the force of the coil spring 79, as shown in FIG. 6(a), the low speed rocker arm 74 is uncoupled from the high speed rocker arm 75. This causes the valve to be opened and closed by the low speed cam by way of the low speed rocker arm 74. This decreases the valve lift.
The valve lift is generally changed based on the engine speed. For example, when the engine is running at a lower speed, the valves are opened and closed by the low speed cam as illustrated in FIG. 6(a) to decrease the amount of air drawn into the engine. When the engine is running at a higher speed, the valves are opened and closed by the high speed cam to increase the amount of air drawn into the engine.
An oil passage 85 is connected to the oil pressure passage 72 for supplying oil to the passage 72. The supplied oil is used to lubricate the low speed and high speed cams. The sliding surfaces between the cams and the rocker arms 74, 75 also need lubrication. The passages 72, 85 are connected to an oil pump 83 via a switching valve 87. The switching valve 87 includes a variable orifice 86 and is connected to an oil pump 83. The oil pump 83 is driven by a crankshaft of the engine (not shown). The pump 83 draws oil from an oil pan 84 and discharges the oil to the switching valve 87.
When the engine is running at a high speed, the switching valve 87 sends oil from the oil pump 83 to the oil pressure passage 72 as illustrated in FIG. 6(b). The oil is then flows to the passage 85. In this state, the restriction amount of the orifice 86 is controlled to deliver enough oil to the chamber 78 to displace the pin 77 against the force of the spring 79 as shown in FIG. 6(b). Thus, the oil pressure actuates the mechanism 71 and switches the cams for increasing the valve lift. Part of the oil passing through the oil passage 85 is injected from holes 88 for lubricating the sliding parts of the cams and the rocker arms 74, 75.
When the engine is running at a low speed, the switching valve 87 sends oil from the oil pump 83 to the oil passage 85 as illustrated in FIG. 6(a). The oil then flows to the passage 72. In this state, the restriction amount of the orifice 86 is controlled so that the oil pressure in the chamber 78 is too low to displace the pin 77 against the force of the spring 79. As a result, the mechanism 71 switches the cams to decrease the valve lift. Part of the oil passing through the oil passage 85 is supplied to the cams for lubricating the sliding parts of the cams and the rocker arms 74, 75.
However, when the engine is running at a low speed, the power of the oil pump 83, which is driven by rotation of the crankshaft, is also lowered. This results in less oil being discharged from the pump 83. At low speeds, oil is supplied to the oil pressure passage 72 via the oil passage 85. In other words, after part of the oil in the passage 85 is diverted to the sliding parts, the remaining oil flows to the oil pressure passage 72. Accordingly, the oil pressure in the passage 72 is lowered. Thus, when the engine is running at a low speed, it takes a significant amount of time to generate enough oil pressure in the oil passage 72 to actuate the mechanism 71 in the oil passage 72. Under these conditions the mechanism 71 has a relatively slow response.
When the engine is running at a high speed, oil is supplied to the oil passage 85 via the oil pressure passage 72. The pressure of the oil in the passage 72 falls when the mechanism 71 is actuated. Accordingly, the oil pressure in the oil passage 85 is lowered. This reduces the amount of oil supplied to the sliding parts. Thus, the lubrication of the sliding parts may be insufficient.